Real-time s-parameter imager

ABSTRACT

Disclosed is a fully automated system capable of producing high quality real-time S-parameter images. It is a useful and versatile tool in Material Science and Solid State Technology for determining the location of subsurface defect types and concentrations on bulk-materials as well as thin-films. The system is also useful in locating top surface metallizations and structures in solid state devices. This imaging system operates by scanning the sample surface with either a small positron source ( 22 Na) or a focused positron beam. The system also possesses another two major parts, namely electronic instrumentation and stand-alone imaging software. In the system, the processing time and use of system resources are constantly monitored and optimized for producing high resolution S-parameter image of the sample in real time with a general purpose personal computer. The system software possesses special features with its embedded specialized algorithms and techniques that provide the user with adequate freedom for analyzing various aspects of the image in order to obtain a clear inference of the defect profile while at the same time keeping automatic track on the instrumentation and hardware settings. The system is useful for semiconductor and metal samples, giving excellent quality images of the subsurface defect profile and has applications for biological samples.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to digital imaging, and more specifically,creating high-quality S-parameter images in real-time.

2. Background

Positrons are known for their sensitivity to vacancy type defects insolids. Using Doppler broadening spectroscopy, the shape of the 511 keVannihilation line, as represented through the S-parameter, provides ahelpful measure of neutral and negatively charged vacancy defects.S-parameter imaging of a wafer surface by laterally scanning a pointsource of positrons across the wafer surface can provide usefulinformation regarding the density of bulk defects in the wafer. Suchscans have been made in the past by moving a radioactive source or apositron beam across a wafer. In this method, the beam or radioactivesource is kept fixed and one point on the sample is exposed to thatradiation for some fixed time and all annihilation events are thenrecorded for that point before moving on to the next point. The nextpoint is then exposed to the positron radiation and so on until thewhole sample surface has been scanned. The whole data set is finallyprocessed off-line as per the requirement and is finally plotted in theform of an S-parameter graph using plotting software.

BRIEF SUMMARY OF THE INVENTION

There are at least four major draw backs of the standard imaging method.The first drawback is that the image can only be visualized after thecompletion of the entire surface scanning, which takes typically a fewdays for storing sufficient information to produce a good quality image.Using this method it is quite possible to collect data for a longer thannecessary period if the S-parameter features of the sample turn out tohave good S-parameter contrast. In some cases of prominent defectprofile, even with less data at each pixel point it may still bepossible to make a nice inference.

The second drawback is that analyzing the image with any graphingsoftware is always static. In such a system, one cannot dynamicallyinteract with the visualization process.

Third, there is no global monitoring of the image during the data takingprocess. For example, the right justification of the sample with respectto the scanning source/beam may be in error. Such an error can only bepicked up after a completed sequential rectilinear scan had been made.Thus, much time can be wasted in improper positioning of the sample.

Finally, even those research groups having a positron beam arerestrained to moving the sample using a mechanical system. This meansthat there will always be a possibility of some mechanical backlash.

In contrast, the present system works preferably by parallel rapid andcontinuous rastering of the whole sample surface, with information ateach pixel location being built up at the same time. Alternatively,sequential point-by-point accumulation of data is used. The presentsystem is supported by imaging software that allows the user to monitorthe image build-up online in a dynamic and interactive mode thatfacilitates the formation of a good quality image of the sample in theshortest possible time. This system is useable with a ²²Na source aswell as a sub-millimeter diameter positron beam. The imager system hasmany more features that will be discussed below.

Disclosed is a real-time automated imaging system for solid statetechnologists that permits the location of subsurface defects on anymaterial sample by reading its positron image. This S-parameter imagingtechnology employs a technique for scanning the sample surface in‘parallel-rapid rastering’ mode. The electronic setup for signaling anddata processing utilizes nuclear instrumentation modules in such a wayso as to optimize the accuracy, precision and efficiency of datacollection. Further, according to one embodiment, the data acquisitionand image processing is performed in real time. This technology has beendeveloped for two major categories of researchers, those that have afocused low energy positron beam and those that do not. With thisS-parameter imager, all the drawbacks of the existing techniquesdiscussed above have been overcome.

The major advantages may briefly be listed as: (i) the rapid andrepetitive scanning of different points of the sample one after anotherfor the whole surface; (ii) the data is processing in real time withconcurrent on-line image construction; (iii) image features can be seenquickly; (iv) electromagnetic deflection of the positron beam permitsfast rastering for more advantageous configuration of a fixed sample;(v) electromagnetic deflection of the positron beam eliminates possiblebacklash of a moving sample stage; (vi) the imaging software permits theuser to monitor various important hardware functions so as to check onthe primary data fidelity and to permit necessary adjustments asrequired; (vii) the software is interactive and user friendly; (viii)the software gives the freedom to the user to monitor all setups andimage quality; (ix) the user can continue to scan the sample untilsatisfied with the image quality; (x) optimal resolution which is a bigfactor in image analysis is also taken care of.

BRIEF DESCRIPTION OF DRAWING

FIG. 1A is a layout of a source and scanning setup together with waferand detector: side view.

FIG. 1B is a top view of the source and scanning setup together withwafer and detector showing motion of the source.

FIG. 2 is a block diagram showing one embodiment of the Electronicssetup of the Imager.

FIG. 3 is a ‘virtual instrument diagram/program’ depicting the systemsoftware written in LabVIEW, a graphical programming language.

FIG. 4 is a depiction of a Front Panel of the LabView given in FIG. 3.

FIG. 5 is a side view of the XY deflection coils.

FIG. 6 is a cross-sectional view of the XY steering coils.

FIG. 7 is a block diagram of one embodiment of the imager as applied tothe low energy positron beam.

FIG. 8A is a photograph of e⁺symbol made up of 0.5 mm thick kapton foiland pasted on the surface of silicon wafer.

FIG. 8B is an S-parameter image of the kapton e⁺symbol depicted in FIG.8A.

FIG. 9A is a photograph of monocrystalline Ni sample.

FIG. 9B is an S-parameter image of the sample described in FIG. 9A.

FIG. 9C is a bar-chart plotting of the S-parameter value versus thepixel number with the data values taken along row 55 from FIG. 9B.

FIG. 10A represents an X deflection signal for the beam applicationaccording to one embodiment of the disclosed device.

FIG. 10B represents a Y deflection signal for the beam applicationaccording to one embodiment of the disclosed device.

FIG. 11 represents one embodiment of a sample holder.

FIG. 12 is a block diagram of the imager setup.

FIG. 13A is a side view of the scanning apparatus.

FIG. 13B is a top view of the scanning apparatus.

FIG. 14A is a LabVIEW VI diagram for the library available sub-vi, DIOCONFIG.vi.

FIG. 14B is a LabVIEW VI front panel for the VI given in FIG. 14A.

FIG. 15A is a LabVIEW VI diagram for the library available sub-vi, DIOSTART.vi.

FIG. 15B is a LabVIEW VI front panel for the VI given in FIG. 15A.

FIG. 16A is a LabVIEW VI diagram for the library available sub-vi, DIOREAD.vi.

FIG. 16B is a LabVIEW VI front panel for the VI given in FIG. 16A.

FIG. 17A is a LabVIEW VI diagram for high resolution (pixel) to lowresolution image conversion sub-VI.

FIG. 17B is a LabVIEW front panel for the VI given in FIG. 17A.

FIG. 18 (A) is an S-parameter image of GaAs wafer size 3 cm×3 cm.

The image resolution is 128×128 pixels taken with stepper motor stepwidth 3.6 degrees. (B) S-parameter image of GaAs wafer of size 3 cm×3cm. Image resolution is 64×64 pixels and image is taken with steppermotor step width 3.6 degrees. (C) S-parameter values plotted against thecorresponding pixel number in row 29 of FIG. 18 b.

FIG. 19A is the figure ‘C’ is a 0.4 μm Al thin film deposited upon a Cusubstrate.

FIG. 19B is an S-parameter image (of the figure C described in FIG. 23),imaged with a slow positron beam of spot diameter 1.5 mm.

FIG. 20 is the flow chat of the complete system software.

DETAILED DESCRIPTION

A positron scanning setup shown in FIG. 1 comprises a positron sourcefixed to a holder arm that uses a rastering action across a sample. Theresulting annihilation photons are detected by a detector. In oneembodiment, the positron source comprises a ²²Na source encapsulated ina Kapton foil. The source has a strength of approximately 5 μCi andpreferably a diameter of about 0.5 mm. In one embodiment, the sourceholder angle is approximately 60 degrees from the sample's normal so asto avoid any possible walling above the source where the positrons mightbe reflected back towards the sample or annihilate in other materialother than that of the sample.

In a first embodiment, the holder has a square hole, while in otherembodiments the hole can be rectangular, oval, round, etc. There is athin plastic film/self adhesive tape/cello-tape fixed across the bottompart of the hole in angle holder, as shown in FIG. 11. Preferably, theself adhesive tape has a 1 cm diameter hole.

As discussed above, the source is encapsulated in a kapton foil. Thekapton piece is fixed below the self adhesive tape, on the paste side,so the source (black spot) is kept at approximately the center of thehole in the self adhesive tape. In one embodiment, the source is fixedaround the center of the hole in the self adhesive tape and fixed to theholder such that its bottom part is free of any obstruction while nothindering the downward moving positrons. Also, the hole gives freedom toall upward moving positrons to disperse and annihilate at some distancefrom the detector, rather than being scattered/reflected towards thesample. Preferably, the source moves to about 0.5 mm above the samplesurface to avoid spreading of the positrons. A high purity germanium (HPGe) detector is attached at an adjustable distance (typically 5 cm)below the sample in accordance with required data rates. The distance ismeasured between the sample surface and the axis of the cylindricaldetector head. In another embodiment, the sample and the detector arekept firmly fixed in their relative positions.

Raster scanning of the source is accomplished by driving a dual steppermotor system controlled by software. While the software may be writtenin Visual Basic, other languages or other object oriented languages canbe used.

The system preferably has a resolution of 0.9 degrees in each step and400 steps per second on a screw drive channel rail with diameter 1.24cm. [pitch=1 mm, diameter=12 mm].

The electronics used for signal processing comprises a spectroscopyamplifier, Timing Single Channel Analyzer (TSCA), linear gates, andLVDTS. The spectroscopy amplifier is selected to filter the chargeintegrated pulse outputted from the detector's preamplifier. The TSCAprovides a TTL logic pulses every time a peak amplitude voltage pulsefrom the spectroscopy amplifier falls within a narrow band of voltages(energy window) that signify a 511 keV (annihilation event) gamma rayenergy event. In one embodiment, two linear gates are switched everytime a TTL “hi” is supplied from the TSCA so as to produce the positioncoordinate signals x and y that have pulse heights proportional to the xand y displacements. This allows for efficient management of processingtime and memory by providing x and y coordinate information when anannihilation event triggers the TSCA/detector system. When anannihilation event triggers the detection system, recalculation of theS-parameter occurs for the pixel that has received an event, since ifthere is no event, nothing is contributed towards the S-parameter valueof that concerned coordinate. In one embodiment, the instrumentationsetup includes linear variable differential transformers attached to thescanning instrument as shown in FIG. 13, such that for the whole rangeof movement, they operates with a positive output as required to producepositive x and y pulses for the nuclear ADC. The LVDTs are preferablycalibrated in the range, +5 volts with resolution 0.001V. The scanningapparatus shown in FIG. 13 comprises a rigid iron frame structure onwhich the stepper motors that drive the scanning in the xy plane throughscrew channel rails are mounted. While the system was discussed usingTTL logic, the system can be implemented using CMOS logic, ECL logic, orthe like.

The scanning apparatus comprises two stepper motors which are controlledby a computer running the motion control software program. In oneembodiment the program is written in Visual Basic. Other programs can beused including Delphi, C++, Java, etc. The motion control program istypically run in the environment of the motor control software that isprovided by the manufacturer of the motor.

FIG. 20 depicts the imaging software in flow chart form. FIG. 3 is adepiction of the system software program written in LabVIEW (LabView isa graphical programming language software. In LabVIEW terminology, sucha graphical program is called as ‘virtual instrument diagram’) Theinstrumentation comprises data acquisition, data processing,computations, and real-time image visualization. The software algorithmaccesses binary data from three different nuclear ADCs employed for x, yand energy channels and then converts the 14 bit data blocks (as the ADCoutput is 14 bit binary) into an integer decimal numbers.

In one embodiment, the program is adapted to accommodate the x and ycoordinate data in a 32×32, 64×64, 128×128, or 256×256 matrix. Thematrix is transformed into coordinate pixels in the graphing plane. Bydefault, the system stores data in a 128×128 matrix. But the user mayalso choose it to 256×256, 64×64 and 32×32 by adjusting the rotatableknob named “max resolution” on the front panel (FIG. 4). An algorithm insub-vi form in the imager checks lower pixel images converted from thehigher pixel original image (pixels chosen by the user in “MaxResolution”) acquisition which is a state of continuing acquisition(FIGS. 17 a and 17 b are the VI and front panel of this sub-vi).

The program preferably includes a scaling algorithm using simpledigital-offset. By dividing the remaining value with a fixed number,which is determined according to the maximum and minimum x and ychannels present in the incoming numbers from the front panel shown inFIG. 4, so that the input x and y coordinate values range from 0 to theuser input resolution value for correct indexing to the correspondingdata storage matrix. Included in the program is a provision to monitorthe incoming energy value as shown in the front panel, Energy Channelwindow in FIG. 4 from which the user can select maximum energy value.

The program also monitors the energy spectrum in a xy plot having y axisas the total sum of all energy values stored for a particular x and yindex and the subsequent pixels, in Physics term, channel no, in thesame row of the matrix. The energy spectrum is plotted giving a veryefficient way to monitor the electronic drift during the period of imageacquisition or a portion thereof. The program includes a provision forthe user to input a numbered which will be deducted, a digital offset,from each of the incoming energy values. This deduction is to keep only100 channels around the peak channel (the maximum energy value) forprocessing. This input number is inputted by monitoring the energyspectrum on the front panel as shown in FIG. 4.

Additionally, a half width value having a decimal value having 3 digitsafter the decimal point can be entered as shown in the front panel FIG.4. The number is selected to preferably keep the “Mean S” value veryclose to 0.500 (front panel, FIG. 4) which is the optimal forS-parameter sensitivity. The peak channel is measured using the energyspectrum graph as displayed on the front panel, as shown in FIG. 4. Thisvalue is determined by an automatic peak finding method involvingfitting a 2^(nd) order polynomial to the top 25% of the spectral height.The user inputs the half width value that is used to compute theS-parameter, and, after a few trials, achieves a value that produces anS-parameter of around 0.500. Once the half width value is fixed, thepeak position is checked (by 2^(nd) order polynomial fitting) in acertain time interval (use optional between 1-10 minutes) to monitor theelectronic drift and an automatic feedback loop ensures that the centralregion is kept locked on to the peak center.

This minimizes computational time as well as the memory occupancy asthere is no excessive real-time computational procedure. The S-parameteris computed in real time in optimal system time consumption and theoptimal memory occupancy. The S-parameter is computed by taking the sumof all energy events coming in the range of the peak channel+half widthand the peak channel—half width to the sum of all energy events comingin the window of 100 channels around the peak channel, i.e., peakchannel −50 to peak channel +50.

The optimization of the system is accomplished by keeping control overthe image refreshment period (‘Image Refresh’ window, front panel, FIG.4) so that the user can input the refreshment period by inputting aninteger number which is taken as milliseconds. Refreshing 10 times in asecond is almost like real time refreshment as the human eye cannotdetect changes at a much faster rate. In other embodiments, the refreshrate is set automatically. The S-parameter image (S-parameter Imagewindow, FIG. 4) is plotted in an intensity plot such that the length andbreadth of the sample are plotted in x and y axis respectively and theS-parameter value is taken in the third dimension and such that a colorscale corresponds to the S-parameter value.

The real-time visualization of the S-parameter image, the resolution ofwhich depends upon the matrix size (number of pixels) which is assignedfor the data storage in order to get the various pixels of the image.Additionally, the real-time visualization of the S-parameter image withthe resolution of this image depending upon the step width of thestepper motors employed for the x, y motion with less step width givingbetter resolution.

In yet another embodiment, the real-time visualization of theS-parameter image where the time required to get a decent image dependsupon the rate of annihilation events detected by the detector and thentransferred to the imaging software with higher event/data ratesproviding faster image production. A diagnostic rate-meter tool (FIG. 4)is provided which shows the average rate of incoming annihilation eventdata in the range of 100 channels around the peak channel. Themonitoring of which gives an estimate of the efficiency by which eventsdetected by the detector are being successfully transferred in to theimaging software.

With use of the instrumentation technique, as depicted in FIG. 7, thesystem can also take an image using a monoenergetic positron beam (itmeans a particular energy setting for positrons in case of variableenergy beam) substituting the ²²Na source. As shown in FIG. 7, theinstrumentation technique comprises x and y deflection coils coupled tosignal generators that provide suitable ramp voltages to the x and ydeflection coils. The charged particle beam can be deflectedelectromagnetically so as to raster the positron beam across a specifiedarea of the sample.

The x and y coil pairs which are wound with 22 gauge wire are shown inFIG. 5 and FIG. 6 on a proper frame in the appropriate shape. It shouldbe noted that shape and dimensions depend upon the beam design structureof the user with the coils being made firm with application of some glueand then being mounted to the beam structure with suitable support.

The xy coils are calculated to meet the current supply capacity toproduce the necessary deflection magnetic field strength, a calculationwhich can easily be done using Ampere's current law or the Biot-Savart'slaw. The x and y signals which are respectively a uniform triangularwave (frequency=50 Hz, Amplitude=5V) and a saw-tooth ramp voltage(frequency=1 Hz, Amplitude=5V). The x and y signals which are currentamplified up to a suitable level as necessary for the deflectionmagnetic field necessities which can easily be calculated with help ofAmpere's current law.

A first embodiment of the disclosed S-parameter imaging system consistsof three major sections, namely the ²²Na source scanning apparatus, thepulse processing electronic setup, and the dedicated system software.The imager system is developed using a radio active positron sourcewhich is a focused low energy positron beam. The basic imager isdescribed as well as the positron beam application.

The sources and scanning apparatus are shown in FIG 1. In oneembodiment, the source is comprised of a 0.5 mm diameter 5μCi²²Naencapsulated in 8 μm thick Kapton foil. The source is supported by athin plastic sheet attached to an aluminum support frame. The aluminumsupport frame is connected to an X-Y stepper motor drive. The source issuspended 0.5 mm above the wafer surface. Other types of source designs,such as ones involving collimation of positrons by Al or heaviermaterials that may allow some screening of annihilations coming directlyfrom the source material itself or a source with no backing material andno collimation. Positrons ejected in the direction opposite that of thewafer traveled tens of cm in the air and the fraction of annihilationsreceived from such positrons was thus minimized through the inversesquare law. During the scanning motion the distance from the source tothe center of the detector varies between 5 and 6 cm causing slightvariations of count rate which have no significant effect on thedetectors energy resolution.

The source is moved in rectilinear motion (see FIG. 1) by stepper motorswhich operate linear screw drives. The stepper motor drives arecontrolled by an independent computer. Linear Variable DifferentialTransformers (LVDTs) are attached to the base of the support frame so asto provide voltages proportional to both x and y motions.

Annihilation photons, predominantly from the wafer, are detected usingan HP Ge detector. Preferably, the detector has a 20% efficiency. Thedata rate into the annihilation line is preferably limited to 1,500positron annihilation events per second after optimizing the detector tosource distance. Components for one embodiment are shown in Table 1

FIG. 2 depicts a block diagram of one embodiment of the electronicinstrumentation. As the positron source rasters across the wafersurface, the x and y LVDTs provide DC voltages proportional to thedisplacement in these direction. These voltages are converted to pulsesof 2 μs is width using two linear gates that are fed from a TimingSingle Channel Analyzers (TSCA) output, which in turn is derived fromannihilation photons that give energy signals in a narrow window around511 keV. It should be noted that every energy event within the definedwindow there is an associated x and y pulse, the amplitude of whichgives the position of the source at the time of the event. Theannihilation photon energies E_(γ)together with the x and y pulsesignals are processed using a 14 bit nuclear ADC, a peak search and holdADC. Here the use of the TSCA and the Spectroscopy Amplifier (S.Amp) areas per the standard nuclear instrumentation setup.

Using event data triplets (x, y, E_(γ)) an S-parameter is computed inreal time for each pixel region and is used it to refresh a color imagedisplay on the screen coordinates. The program is written using LabVIEW6.1. FIG. 3 shows the virtual instrument (VI) diagram that depicts theblock diagram program of the whole software part of the system. This VIconsists also of some sub VIs that are available in the library of theLabVIEW software. DIO CONFIG.vi, DIO START.vi and DIO READ.vi are thelibrary subVIs used here for digital input port configuration, timing ofdata sampling and data reading respectively. FIGS. 14 a & 14 b, 15 a &15 b and 16 a & 16 b show the detailed virtual instrumentation diagramsand front panels with typical parameter sets respectively for the abovesaid subVIs. Three identical modules are employed to read 14 bit binarydata from the x, y & E (energy) channels. From the output of the subVIDIO READ, the 14 bit binary stream is then passed through furtherprocessing in order to convert it into numbers as shown in FIG. 3. The xand y voltages pulses (spanning 0-5V) are digitized into a 8k numberwhile the E voltage (0-10V) is digitized into a 16k number. The numbersfrom these three different ports are scaled, manipulated, processed andfinally traced out into the S-parameter image.

After scaling the three primary inputs (x, y, and E), the data is storedin two 2-dimensional arrays. The first is referred to as the C array andcounts events that have fallen in the center region (defined by thecontrol “half-width” in FIG. 4) of the annihilation peak. The second isthe T array which counts all annihilation events irrespective of wherein the annihilation peak they fall. Each event is indexed into theappropriate element of the array according to the value of x and y forthe event. The final stage is computing the S parameter for each arrayelement. With reference to the FIG. 3, the computational strategy forreal-time determination of S-parameter is preferably optimized in timeand memory consumption. The scheme of S-parameter computation is made aspracticable as possible by considering the empirical values, unlike thetraditional method in which S is calculated in an extremely bulky datahandling and computational procedures involving background reduction,peak fitting (i.e., spine or polynomial fitting) in real-time. Inpractical observation, these extremely heavy computational expenses donot give any noticeably improved sensitivity with regards to minuteinference of defect characteristic in image analysis to make themwarranted. In contrast, the S-parameter is simply obtained by dividingthe sum of events in the central region “C” by the total sum of events“T” coming in the specified 100 channels, for all array elements (S=C/Tby definition). This array of S-parameters is stored and forms theS-parameter image.

The imaging software has an embedded important facility that allowsvariable lateral resolution of the image. There are 4 differentresolution levels, namely 32×32, 64×64, 128×128 and 256×256. These areuseful in that it takes time for S-parameter information to build up inthe image. The accuracy of the S-parameter in each pixel depends on thesquare root of the number of events for the pixel (or the square root ofthe amount of accumulation time). For this reason, the user may chooseto start the imager with a 32×32 resolution and increase this to 128×128as time goes on. Indeed sufficiently good images may be achieved,according to user requirement, for example with 64×64 resolution. Thenumber of pixels in the plot is a user defined option. FIG. 4 shows thecontrol panel in which the rotatable knob “Max Resolution” serves thepurpose of resolution variation. The changing of resolution isaccomplished using an algorithm in sub-vi form to check the lowerresolution image converted from the higher resolution image (128×128)acquisition which is continually accumulating (FIGS. 17 a and 17 b arethe VI and front panel of this sub-vi). In FIG. 3, it is seen that x andy incoming values serve the x and y indices in order to accommodate theincoming parameter value in the 2D array mapping. Preferably, theminimum and maximum values of the incoming x and y values should rangefrom about 0 to about 128. To achieve this, the x and y digitizedvoltage values from the LVDTs undergo both a digital offset and scaling.This may be seen from the front panel (FIG. 4), where the maximum andminimum incoming values of the x and y ports are noted and thedifference of the respective max and min values is divided so as toproduce natural numbers up to about 128 after being rounded off. Thereis preferably no scaling for the energy values, but much of the data isdiscarded since it does not form part of the annihilation line. Thusonly those digitized energy values that lie within about ±50 of theannihilation peak maximum or peak channel are used for the S-parametercalculation. The shape of the annihilation line ‘spectrum’ can be viewedfrom the front panel (FIG. 4), a suitable number being manually enteredby the user so as to keep only those 100 channels around the peak valuewhich will give rise to the average S-parameter value approximately0.500 in view. Also only the positive energy values are being chosen.The user has to enter the value of the ‘half width’ (of the 511 Kevevent) in the specified field (6 digit decimal) of the front panel (FIG.4) so as to keep the average S-parameter around 0.500 (front panel, FIG.4).

Also as part of the major time management scheme, the image refreshingperiod for the image visualization on the screen is user defined. FIG. 4shows a user input field, ‘Image Refresh’ whose value serves the timebetween the successive refreshments, preferably in milliseconds. Thespectrum is also refreshed in a time interval of 1-10 minutes (userinput in ‘Spectrum refresh’ field of the front panel FIG. 4). In factthe quicker spectrum refresh does not provide any noticeable change inits shape.

For monitoring the system performance, there are several monitors on thefront panel, FIG. 4. The ‘rate’ meter is one of them. This shows theaverage rate of annihilation peak events “T” coming inside the ±50channel range. It gives an indication of the number of successfullyprocessed annihilation peak events from which the efficiency of thesystem can be worked out since the annihilation line event rate from thedetector can be monitored separately by single channel analyzer (SCA)and a rate meter. In one embodiment, there are also ‘scan back log’monitor windows on the front panel which shows an increasing trend (bynumber display) if all data coming to the data acquisition card bufferis not transferred into the imaging software. Preferably, this buffershould be kept constant with a small number for example, 2 digits. Thismay be achieved by setting the ‘buffer size’, which is another controlwindow in the front panel. The settings of other parameters such asimage refresh rate, matrix dimensions, and computational jobs influencethis parameter. The spectrum plot, which can refresh about every 10seconds, also helps in monitoring any electronic drift that occursduring long term accumulation.

FIG. 8A shows photograph of the symbol “e⁺” made up of 0.5 mm thickkapton (polyimide) foil pasted on the surface of silicon wafer. TheS-parameter image taken by the invented imager with 128×128 pixelresolution is shown in FIG. 8B.

FIG. 9A is the photograph of a monocrystalline Ni sample produced byhigh pressure torsion. The sample was produced by high pressure torsion.This means that the defect profile should be a radial distributionaround the center of the sample. FIG. 9B is the S-parameter image of thesample where the parametric values are prominently distributed radiallyaround the center making clear that that the defect profile is alsodistributed radially about the center. To draw a clearer inference fromthis S-parameter image data set, the user may take the S-parametervalues along a row of pixels passing through the central region of theimage. FIG. 9C is the bar-chart plot of the above mentioned taken alongrow 55 of FIG. 9B. The height distribution of the bars which stand forthe S-parameter values of the corresponding pixels is graduallyincreasing from centre of the sample towards the periphery. This is aclear inference of the defect profile as was predicted by the samplesupplier. The image in FIG. 9B is taken with stepper motor step width of3.6 degrees in the y-direction which causes the observed vertical“striation”. The step width of the motors could be reduced (to 0.9degrees minimum) in which case a smoother image of the sample would beformed.

FIG. 18 shows two S-parameter scan images taken for n-type GaAs—Sidoped—with carrier concentration of 10¹⁷ cm². FIG. 18A shows a 128×128pixel resolution plot. There is a good deal of “granulation” in theimage. Much of this comes from the poor statistical accuracy availableon the S-parameter (stand. dev. ΔS=±0.004)—with only 15,000 events perpixel region. On the other hand in FIG. 18( b) the spatial resolution isdecreased to a 64×64 pixel resolution plot and there is an improvementin image visualization produced through the decreased statistical erroron S (ΔS=±0.002). Because of the natural spread of positrons from thesource (˜1 mm), it can be argued that this is an optimal resolution forscanning a 5×5 cm region—since the source spread does not permit anyfiner pixel region division. It is noted that FIG. 18B shows clearly thepresence of some “hot” (white) regions that indicate the presence ofregions with a higher concentration of defects. There are also someregions of the image where the image is darker than average and thesemay be interpreted as regions of low defect density.

In order to show more clearly the presence of defects in the wafer theS-parameter data of a single row is plotted in FIG. 18C. This is row 29of the FIG. 18B image. As may be seen, the peaks and valleys are far inexcess of the statistical allowance. Using S-parameter alone it isdifficult to tell the defect type being mapped. In n-type, GaAs bothnegatively charged As and Ga vacancies are known to trap positrons.However the difference between maximum and minimum S-parameters is˜0.04, which is more than that expected for saturation trapping intomonovacancies. In another embodiment of the system, the imager systemuses an R-parameter in addition to the S-parameter.

The imager system as described finds easy extension to use with afocused monoenergetic positron beam. Here the term “focused” means thatthe beam diameter at the target must be 1 mm or less. This focused beamspot essentially replaces the small diameter positron source. The xyscanning by the dual stepper motor system is replaced by a rasteringmotion of the positron beam, which in turn is achieved by thedisplacement of the charged particle beam by appropriate application oftime-varying (saw-tooth on y and triangular on x) magnetic fieldssupplied by x and y deflection coils. FIG. 7 is the block diagramdepicting this complete plan of the S-parameter imager as used with apositron beam system. FIG. 5 and FIG. 6 give the side andcross-sectional elevations of the xy steering coils. Due to the strongaxial magnetic field normally used in low energy positron beams the mostappropriate form of deflection is adiabatic deflection in which a smallsideways magnetic field causes a small change in the direction of netmagnetic field and the positron motion down the beam. The gauge value ofthe wire and number of turns for one of the pairs of steering coils caneasily be calculated by applying Ampere's Law to the coil system of FIG.6, i.e., B={[6.928×10⁻³I]/r} Gauss, where B is the sideways deflectingmagnetic field, I is the current and r is the radius of the xy tube. Inone embodiment, for a beam, in which a deflection field of 1 Gauss isrequired, each coil winding of 100 turns of gauge-22 wire is used. Eachpair of coils has a resistance of about 7Ω which are used in conjunctionwith current amplifiers that can supply up to 1A. FIG. 10 shows the xysignals. The X coils are being fed uniform triangular waves (Freq=50 Hz,Ampl=5V) from a signal generator. The y coil pairs were fed with rampvoltage (Freq=1 Hz, Amp=5V) from another signal generator. All theparameters of the signals can be varied/controlled as per the beamdeflection needs of the user. Also the x and y signals are being fed tothe data acquisition card and ANDed with the amplified output of thedetector as in FIG. 7 so as to keep track of the position coordinates ofeach event.

A positron beam is used with the x, y scanning system. The S-parameterimage taken with this type of slow positron beam is useful for thin filmsamples. FIG. 19A and FIG. 19B show the photograph of the symbol ‘C’(0.4 μm thick aluminum deposited on a copper substrate) and theS-parameter image respectively. The edges of the image are not too sharpand some portions of the image are blurred as the beam spot is 1.5 mmdiameter. The image improves as the beam spot is 0.5 mm or less.Additionally, the presence of some fast positron contamination in thebeam causes blurred images. The removal of the contamination yieldsclearer images. A sub-millimeter beam and removing the fastcontamination yields better quality images. In one embodiment, a micronsize beam spots of mono-energetic positrons is used. If applied inconjunction with such micron-sized beams the imaging system will give anextremely high resolution image of micron sized electronic devicestructures. Moreover such images could be produced within reasonableoperation times with high beam intensity.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the inventiveprinciples, it will be understood that the invention may be embodiedotherwise without departing from such principles. Accordingly, thespirit and scope of the present invention is to be limited only by thefollowing claims.

1-15. (canceled)
 16. An instrumentation setup for processing imagerelectronic signals comprising: a spectroscopy amplifier; an SCA (SingleChannel Analyzer); linear gates; ADCs (Analog to digital converters);and LVDTs (Linear Variable Differential Transformers).
 17. Theinstrumentation set up for processing imager electronic signals of claim16, further comprising a detector and a preamplifier coupled to thedetector wherein the spectroscopy amplifier is adapted to optimallyfilter a charge integrated pulse output from the preamplifier.
 18. Theinstrumentation set up for processing imager electronic signals of claim16, wherein the SCA is adapted to provide TTL logic pulses in responseto a peak amplitude voltage pulse from the spectroscopy amplifierfalling within a narrow band of voltages signifying a 511 keV(annihilation event) gamma ray energy event.
 19. The instrumentation setup for processing imager electronic signals of claim 16, furthercomprising: two linear gates adapted to switch in response to a TTL highbeing supplied from the SCA to produce x and y position coordinateinformation signals having pulse heights proportional to x and ydisplacements.
 20. The instrumentation set up for processing imagerelectronic signals in claim 19, wherein the two linear gates are furtheradapted to provide x and y coordinate information in response to anannihilation event triggering an SCA/detector system.
 21. Theinstrumentation set up for processing imager electronic signals in claim20, wherein a recalculation of an S-parameter for a pixel occurs inresponse to the pixel receiving an event.
 22. The instrumentation set upfor processing imager electronic signals in claim 16, furthercomprising: a scanning instrument, wherein the LVDTs are fixed to thescanning instrument. 23-49. (canceled)
 50. An imager apparatuscomprising: an X-axis frequency generator; a Y-axis frequency generator;an X-axis current amplifier; a Y-axis current amplifier; an HP GeDetector; a spectroscopy amplifier; a single channel analyzer (SCA); afirst linear gate, said first linear gate anding an output from the SCAand an output from the Y-axis frequency generator; a second linear gate,the second linear gate anding the output from the SCA with an outputfrom the X-axis frequency generator; a first A/D converter forconverting the output from the spectroscopy amplifier; a second A/Dconverter for converting the output from the second linear gate; and athird A/D converter for converting an output from the first linear gate,wherein outputs of the A/D converters are input into an S-parameterimaging program.
 51. The imager apparatus of claim 50 wherein the imageis taken using a monoenergetic positron beam.
 52. The imager apparatusof claim 50 wherein the image is taken using a particular energy settingfor positrons in case of a variable energy beam.
 53. The imagerapparatus of claim 50 further comprising x and y deflection coilsconnected to said X-axis frequency generator and said Y-axis frequencygenerator so that a charged particle beam can be deflectedelectromagnetically to raster the positron beam across a specified areaof a sample.
 54. The imager apparatus of claim 53 wherein the x and ydeflection coils are wound with 22 gauge wire.
 55. The imager apparatusof claim 53 wherein the X deflection coils and the Y deflection coilshave a number of turns calculated to meet a current supply capacity inorder to produce a magnetic field strength for a desired deflection. 56.The imager apparatus of claim 50 wherein the X-axis frequency generatoris adapted to produce uniform triangular wave signals, and the Y-axisfrequency generator is adapted to produce saw-tooth ramp voltagesignals.
 57. The imager apparatus of claim 50 further comprising x and ysignals which are current amplified for producing a specified magneticfield for a desired deflection of the positron beam. 58-61. (canceled)